1. Field of the Invention
The present invention relates to an image forming apparatus that controls image formation using a first control unit and a second control unit connected to a load.
2. Description of the Related Art
An image forming apparatus using electrophotography includes functions for image formation, such as a conveying function to convey a recording material such as a sheet, an image forming function to generate an image, and a fixing function to fix an image on the recording material. When those functions are controlled by a single central processing unit (CPU), the processing load of the CPU increases, thus making the overall processing slow. In addition, arrangement of wiring from the CPU to a board where an actuator such as a motor is connected becomes complicated. Therefore, it is desired to execute distributed control on the operation of the image forming apparatus by using a plurality of control modules that achieve those functions. Each control module includes a CPU which executes a process for the associated function (hereinafter referred to as “sub CPU”).
Serial communication is used for transmission and reception of data between the sub CPUs of a plurality of control modules. Each control module controls a load driver such as a motor driver by using the sub CPU incorporated in the control module. An increase in the number of load drivers which the sub CPU controls increases the data communication traffic in serial communication.
U.S. Pat. No. 5,898,666 describes an apparatus that changes the communication rate between the upstream direction and the downstream direction in serial communication.
Japanese Patent No. 4748788 describes an apparatus in which a dedicated line is provided to an optional device in addition to the serial transmission lines.
The known structures to achieve serial communication of an image forming apparatus include a structure which uses two lines of a transmission line and a reception line, a structure which uses two lines of a clock line and a transmission/reception line, or which uses three lines of a clock line, a transmission line, and a reception line. Because an image forming apparatus needs to sheets without jamming, a control sequence for controlling sheet conveying is extremely important. In the case of conveying a recording material such as a sheet of paper, it is necessary to first determine the speed of a motor for conveying the recording material, and then start rotating the motor. However, when serial communication for determining the motor speed is not established due to noise or the like and the motor speed is not determined, serial communication for starting the rotation of the motor may be established before the motor speed is determined. In this case, the motor cannot be rotated at the proper speed, and jamming or the like occurs. To cope with this problem, conventionally, after confirmation of reliable establishment of serial communication, the next communication is started every time. Executing serial communication in this way can permit the control sequence to be executed normally, thus ensuring a proper image forming operation. In order to properly execute the control sequence for the image forming operation, as described above, it is important to check whether or not the communication is established.
In addition, in order to properly execute the control sequence for the image forming operation, information needs to be acquired regularly. In order to control transportation of a recording material with high precision, for example, it is necessary to regularly acquire the output result of a sensor which detects the conveying state of the recording material.
In this case, if communication for checking whether or not communication is established to regularly acquire the information is delayed, control on the image forming operation may not be executed properly.